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Finite toroidal plasma current reduces the magnitude of the normal magnetic field line curvature near mid-volume in a quasi-axisymmetric stellarator. This improves the ideal ballooning stability conditions. Near the edge of the plasma, a hollow toroidal current profile deteriorates the ballooning stability properties by enhancing the normal curvature, while a parabolic current profile improves this stability because the normal curvature decreases. The magnetic field line bending term can also impact the stability properties when the normal curvatures are comparable. The local magnetic shear and the geodesic curvature vanish where the normal curvature is most destabilising. The quasi-periodic ballooning curvature contribution, unlike the normal curvature, fails to constitute a useful indicator about the localisation of ballooning structures.

Additional information

Authors: COOPER W A, Centre de Recherches en Physique des Plasmas, Association Euratom-Confédération Suisse, Ecole Polytechnique Fédérale de Lausanne (CH);OKAMURA S, National Institute for Fusion Science, Toki-shi (JP);YAMAZAKI K, National Institute for Fusion Science, Toki-shi (JP)
Bibliographic Reference: An article published in: Plasma Physics and Controlled Fusion, 43 (July 2001), pp. 839-847
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